In conventional superconducting Nuclear Magnetic Resonance (NMR) magnet coil systems NbTi and Nb3Sn wires are usually used, which limits the field strength of the NMR magnet coil system to approximately 23.5 T. In order to achieve higher field strengths or to provide a more compact magnet coil system, alternative conductor materials may be used. In such devices, “high temperature superconductor” (HTS) strip conductors (for example YBCO strips) are predominantly used. Such magnet coil systems may not be manufactured completely from HTS materials; for reasons of cost it may be advantageous to use HTS materials for the innermost portions and to manufacture the background magnet using conventional “low-temperature superconductor” (LTS) technology (e.g., materials such as NbTi or Nb3Sn).
However, HTS materials may impose special requirements on the quench protection of a superconducting magnet. “Quench” refers to the spontaneous transition of the magnet coil from the superconducting state into the normal-conducting state due to overloading of the current-carrying superconductor. The quench usually starts locally and spontaneously and then propagates in the magnet over several seconds. A quench may be associated with high electrical voltages, currents and forces in the superconductor, which can in turn destroy the magnet.
In a typical quench protection circuit for NMR magnets individual sections or zones of individual sections are connected in parallel with protective resistors and thus form a loop of a protective network. The different protective network loops are connected in series. In this way it is possible to keep the quench duration and quench voltages low (see Wilson “Superconducting magnets”, Chapter 9.8, pages 226ff, 1983, Oxford University Press).
However, typical HTS materials may be disadvantageous in the event of a quench. The high critical temperature of the HTS material (zero field: YBCO approximately 90 K, Nb3Sn approximately 18 K, NbTi approximately 10 K) leads to a late “co-quenching” when the quench starts in the NbTi or Nb3Sn part of the background magnet. Depending upon the quench protection method (for example in the event of sub-division into protective network loops) this results in an increase in current or force in the HTS sections. Moreover, the slow quench propagation in the HTS sections leads to local overheating, which may cause a burnout of the conductor. Therefore, in loops in which the superconductor quenches late, increases in current and force may occur, which may overload the superconductor.
German Patent Application DE 10 2009 029 379 discloses a magnet coil system in which each section that quenches late (for example HTS material sections) is protected in a common loop together with a coil part which quenches fast or early. As a result, increases in current are prevented in the sections which otherwise quench late, thereby preventing increases in current (or burnout) in the HTS portions.
It is known from U.S. Pat. No. 7,649,720 B2 to start a quench by additional heating, in order to avoid increases in current. An improved active quench propagation in the HTS conductor is effected by the use of many extensive heaters in the HTS winding. However, a disadvantage of this is that in the event of a quench the HTS section still risks being destroyed. Moreover, the manufacturing costs during winding of these sections with additional heaters are very high and the homogeneity of the magnetic field generated by these sections can be disrupted by the plurality of heaters, and the associated non-roundness of the winding.